An opening/closing device used in a flow passage such as a sewage system has conventionally been known (refer to Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-300895)), for example). This opening/closing device dams the flow passage while a valve is closed. Then, garbage is accumulated downstream in the flow passage. If the water level in the flow passage exceeds a predetermined water level due to a rainfall or the like, the valve is brought into an open state, water flows to the downstream of the flow passage, and the accumulated garbage can be flown away. In other words, the flow passage can be cleaned.
It should be noted that a float is used to detect whether the water level of the flow passage exceeds the predetermined water level or not (refer to FIG. 1 of Patent Document 1), for example).
Moreover, there is known such a configuration that frame columns are erected on left and right sides of the valve, lock mechanisms are used to lock the valve to the left and right frame columns so as to prevent the valve from opening (refer to FIGS. 5 and 6 of Patent Document 1), for example). In this case, the float and the lock mechanisms are operationally associated with each other, and if the water level in the flow passage reaches or exceeds the predetermined water level, the lock by the lock mechanism is released, resulting in the valve opening. The left and right lock mechanisms are connected with each other in order to simultaneously release the lock in the lock mechanisms respectively provided on the left and right frame columns.
It is further known that, if the water level decreases while the valve is opened, the valve is returned to the closed state by a spring (refer to FIG. 1 of Patent Document 1), for example). In this case, there is provided such a configuration that the force generated by the spring increases in the state in which the valve is open.